1. Field of the Invention
The present invention relates to a ceramic diaphragm structure, and a method of producing the diaphragm structure. In particular, the invention is concerned with a ceramic diaphragm structure having a particularly shaped thin diaphragm portion or portions.
2. Discussion of Related Art
A diaphragm structure has been widely used, for various kinds of sensors, which structure has, for example, a substrate or base with at least one window or opening formed therethrough, and a thin diaphragm plate formed of a flexible material and adapted to close the window or windows thereby to provide a diaphragm portion or portions. In recent years, this type of diaphragm structure has been increasingly used for piezoelectric/electrostrictive actuators. When the diaphragm structure is used as a component of a sensor, the sensor has appropriate means for detecting the amount of bending or flexural displacement of the diaphragm portion of the diaphragm structure, which displacement is caused by an object to be measured by the sensor. When the diaphragm structure is used as a component of a piezoelectric/electrostrictive actuator, the diaphragm portion of the structure is deformed or deflected by a piezoelectric/electrostrictive element formed on the diaphragm portion, so as to raise a pressure in a pressure chamber which is formed within the diaphragm structure.
The diaphragm structure as described above may be produced by forming an integral body which consists of the substrate serving as a base or support, and a film member which is supported by the substrate and which provides the diaphragm. For improved operating reliability, and increased heat resistance and corrosion resistance, it has been proposed to form the diaphragm structure as a fired integral ceramic body, for use in a pressure sensor or a piezoelectric/electrostrictive actuator, as disclosed in U.S. Pat. No. 4,894,635 (corresponding to JP-A-63-292032) owned by the assignee of the present application, and JP-A-5-49270 filed by the present assignee.
The integral ceramic diaphragm structure as described above is generally formed by laminating a ceramic green substrate having a window of a suitable shape, with a thin ceramic green sheet for closing the window, to thus form an integral green laminar structure having a diaphragm portion which closes the window at one of its opposite openings. The green laminar structure is then fired into the integral diaphragm structure. The inventors of the present invention found that the diaphragm portion, which is formed from the ceramic green sheet at its portion located above the window of the ceramic green substrate, may be concavely deformed or dented, or suffer from cracks, during the firing operation. The denting and cracking at the diaphragm portion are detrimental to the intended functions and operation of the diaphragm, resulting in reduced operating reliability.
The ceramic diaphragm structure as described above is normally provided with a flat diaphragm portion or portions. However, it is difficult to significantly increase the natural resonance frequency of such a flat diaphragm. Further, the flat diaphragm does not exhibit sufficiently high mechanical strength, thus making it difficult to reduce the thickness of the diaphragm. Moreover, electrode films, piezoelectric/electrostrictive film and others formed on the flat diaphragm portion cannot be satisfactorily sintered.